Recently, in the field of medical industry and drug delivery system, intensive research and development has been performed to develop a target-directed drug carrier that utilizes a sol-gel transition phenomenon of hydrogel prepared from an amphiphilic polymer having a hydrophobic group as well as a hydrophilic group.
U.S. Pat. No. 4,942,035 discloses the use of a copolymer of polyalkylene glycol, as a hydrophilic polymer, with polylactide, polyglycolide or polycaprolactone, as a biodegradable polyester polymer, for improving the problem of non-degradability of a so-called pluronic gel (polyethylene glycol and polyethylene oxide-polypropylene oxide-polyethylene oxide block copolymer) in the body.
Additionally, U.S. Pat. No. 5,476,909 discloses an A-B-A type triblock copolymer as a biodegradable polyester polymer, wherein the hydrophobic block (A) is limited to polylactide (PLA), polyglycolide (PGA) and copolymers thereof, and the hydrophilic block (B) is limited to polyethylene glycol (PEG) and derivatives thereof.
Meanwhile, Korean Laid-Open Patent No. 2000-0012970 discloses a pH-sensitive polymer containing a sulfoneamide group and a method for preparing the same. More particularly, the Korean Patent discloses variations in the solubility of a linear polymer formed by random copolymerization of a sulfoneamide monomer with dimethylacryl amide or isopropylacryl amide, or the swelling degree of the crosslinked polymer thereof.
The aforementioned prior art utilizes a block copolymer of a hydrophobic biodegradable polymer with a hydrophilic polymer, which shows a sol-gel transition phenomenon depending on temperatures. More particularly, when the block copolymer is injected into the body in the form of an aqueous solution present in a sol state, it undergoes a transition into a gel state, so that it can be used as a release-controlled drug carrier that carries a drug stably in the body and releases the drug gradually. However, when such temperature-sensitive block copolymers having sol-gel transition characteristics are used, there is a problem in that the block copolymers may cause gelling before they are injected completely into the body, because injection of the block copolymers into the body generates a thermal equilibrium state between the injection needle temperature and the body temperature due to the body temperature. Additionally, although it is reported that the hydrophobic part comprising PLA, PLGA or PCL shows pH sensitivity, actual pH sensitivity of the hydrophobic part is not so high as to be applied to the pH condition in the body. Therefore, the block copolymers according to the prior art are not suitable to be applied for drug delivery systems.